In radio receivers and transmitters, an antenna is provided to enable transmission and reception by electromagnetic radiation of radio signals. Various types of antennas exist, with different antennas having advantages for given applications.
As an example, antennas such as monopole and dipole antennas may be formed using one or more wires (respectively) to enable both radio reception and transmission. A dipole antenna may typically include two conductors each having a length that is a quarter of a wavelength, i.e., λ/4, of a desired frequency of operation, in which the midpoint between the conductors is driven by a source to transmit radio frequency (RF) signals at the desired frequency. The conventional dipole antenna generally has a radiation pattern having two generally figure-eight-shaped electromagnetic fields extending around the conductors.
Other applications may use a monopole antenna in which a single conductor is present, along with a conductive plate such as a ground plane that may be adapted perpendicular with respect to the conductor. This type of antenna is driven between the conductive plate and the conductor. Such an antenna results in a resonant structure that generally acts as a half dipole.
Other implementations may use a non-resonant antenna, such as a so-called short monopole antenna, which can be used in a portable device. This short monopole antenna, which is typically formed using a wire, has an electrical length than can be much less than a quarter wavelength of a given radio frequency. However, design limitations exist on such an antenna. For example, the antenna must be distanced from a ground plane, as well as other circuitry of a circuit board that includes a radio receiver or transmitter, to avoid capacitively loading the antenna. Furthermore, performance is less than ideal with such an implementation. That is, because electromagnetic fields associated with the antenna will terminate at the ground plane, the wire antenna must be kept as far as possible from the ground plane. Thus with an integrated antenna on a circuit board, excessive space is consumed in keeping the wire antenna away from the ground plane. Even with such a design, performance is impacted by the relatively close proximity of the antenna to the ground plane.